Polymeric compositions can be obtained in various ways. Curing mechanisms known in the art include radical polymerization, cationic polymerization and anionic polymerization. The curing reaction can be started by different means including heat, radiation or simply by mixing two reactive components containing redox-active substances.
Besides other areas curable compositions can also be used in the dental field, e.g. as dental impression materials.
Dental impression materials are used to record the oral situation of a patient. The resulting hardened impression material captures the negative of the oral situation.
Most dental impression materials are typically delivered in a two or more paste form, containing a base paste and a catalyst paste, which are mixed prior to their application. The mixed pastes are typically applied with the help of a dental tray and/or a syringe-type device. Usually the hardened material can be removed after about one to about six minutes after application. The hardened impression material is used either for making a provisional restoration using a temporary crown and bridge material or for producing a positive model of the oral situation by casting the mould with e.g. gypsum. The obtained positive model is used for making the final restoration in the dental laboratory.
Different types of chemistry can be employed to formulate impression materials. Often used are polyether impression materials which cure by a cationic ring-opening polymerization of aziridines (e.g. Impregum™, 3M ESPE), polysiloxanes which cure via a hydrosilation reaction (e.g. Imprint™, 3M ESPE), polysiloxanes which cure via a condensation mechanism (e.g. Xantropren™, Heraeus Kulzer), mixtures of polyethers and siloxanes which cure via a hydrosilation mechanism (e.g. Senn™, GC) and polyethers which cure via a condensation mechanism (e.g. P2™, Heraeus Kulzer).
Aziridino moiety containing components are typically cured by using strong acids.
Strong acids which can be used include substances like sulfonium salts, especially alkyl sulfonium salts or sulfonium salts derived from glutaconic acid. Those and others are described e.g. in US 2008/0200585 A1, U.S. Pat. No. 4,167,618 and US 2003/0153726 A1.
Aziridine containing prepolymers are used i.a. in the dental field, especially for producing dental impression materials.
The curing reaction of the aziridino moieties containing impression material is typically started by mixing a base paste containing the aziridino moieties bearing prepolymer and a catalyst paste containing a strong acid, especially a Lewis acid.
In this respect, US 2004/0149164 relates to a mixture of elongated N-alkylaziridine prepolymers which can be used as a dental material. The mixture can contain various modifiers like finely divided fillers, pigments, thixotropic agents and surface-active substances.
U.S. Pat. No. 6,599,960 relates to storage-stable cationically polymerized preparations with improved hardening characteristics. The preparations can contain 0.0005 to 50 wt.-% of soluble and/or fine-particle organic and/or inorganic alkaline earth and/or alkali metal compounds. The preparation can be used for making dental impressions.
U.S. Pat. No. 3,926,636 relates to a light-curable composition consisting of a substance containing at least two aromatic or heteroaromatic o-nitrocarbinol ester groups of a certain structure and a compound having at least two aziridine groups or isocyanate groups. The light-curable composition is particularly suitable for the production of coatings and printing plates.
EP 0758 662 A2 (Rockwell) describes a curable epoxy composition containing aziridine and a catalyst to promote curing at ambient temperature. Four classes of catalysts are described. Curing is effected by mixing the curable epoxy resin system with the catalyst. The epoxy resin system is said to have utility as adhesive and coating.
U.S. Pat. No. 4,167,618 (Schmitt) relates to a polymerization process for aziridine compounds. The polymerization process includes mixing an aziridine compound with an alkyl sulfonium salt.
EP 1 431 315 A2 (Tokuyama) relates to a polymerization initiator capable of polymerizing both a cationically polymerizable monomer and a radically polymerizable monomer. The photopolymerizable composition is said to be useful in the dental field, especially as a dental adhesive or as a dental filler restorative.
Especially from a practitioner's standpoint of view it would be desirable to have a material at hand, which can be cured on demand, especially a material which can be applied on a certain surface, adjusted according to the practitioner's needs, and cured when desired.